Genomic analysis and transfer capabilities of Rhizobium grahamii replicons

MARÍA JULIA ALTHABEGOITI; ERNESTO ORMEÑO-ORRILLO; GONZALO A. TORRES TEJERIZO; ANIBAL LODEIRO; ESPERANZA MARTINEZ-ROMERO

Conferencia; 11th European Nitrogen Fixation Conference; 2014

Rhizobium grahamii belongs to a new group of rhizobia together with Rhizobium mesoamericanum. This group has been poorly characterized until now. R. grahamii has a broad-host-range that includes Leucaena leucocephala and Phaseolus vulgaris (bean) although it is a poor competitor for bean nodulation in the presence of Rhizobium etli or Rhizobium phaseoli strains (1). In this work we analyzed the genome sequence of R. grahamii and the transfer properties of its plasmids. Genome sequence was obtained from R. grahamii CCGE502 type strain isolated from Dalea leporina in Mexico (2). The CCGE502 genome was comprised of the chromosome and two plasmids, pRg502a (symbiotic) and pRg502b (chromid). In addition, an integrated plasmid was found in the CCGE502 chromosome. The genomic comparison of chromids and symbiotic plasmids showed that they are species-specific. nod and nif genes encountered in CCGE502 are presented and most of them conserved to those found in R. mesoamericanum. In the ?grahamii? group a single pSym is not prevalent; in contrast, in ?phaseoli? or ?tropici? groups pSyms display higher conservation, around 90% and 99%, respectively (3, 4). We were unable to transfer the R. grahamii CCGE502 pSym and its large chromid to other rhizobial hosts but we could transfer the symbiotic plasmid to Agrobacterium tumefaciens with transfer dependent on homoserine lactones. The conjugative transfer properties of R. grahamii CCGE502?s plasmids may explain their divergence and narrow distribution in related rhizobial species. Most P. vulgaris nodulating bacteria correspond to ?tropici? or ?phaseoli? symbiovars and those pSyms have a high frequency of transfer among rhizobia. Also the ?phaseoli? pSym is found in distantly related species such as R. gallicum (5). The genetic basis of R. grahamii transfer properties should be further explored